If you’re anything like my family, you’ll hop into a car this holiday season, and cruise the neighborhood to view Christmas lights. Who knows? Maybe a house will have decked its walls with a synchronized display to the Trans-Siberian Orchestra’s “Wizards in Winter.”

Or maybe it’s not as elaborate.

In either case, Christmas lights emit some of the magic of the holidays.

Sometimes, usually on an older string of lights — those fat nine-watt incandescent bulbs — you’ll see a halo around the light.

What causes that halo?

Two Types of “Fractions” that Produce Halos

To best understand the halo effect, let’s start with two types of “fractions”: refraction and diffraction. Both definitions involve light traveling through matter.

Refraction

In refraction, light passes through matter and is knocked off course, so it travels indirectly from Point A to Point B.

This is what happens with 22-degree halos, which are the most common halos to form around the sun or moon. The light from the sun encounters ice crystals in the air, and the light is knocked off course, forming a halo around the sun or moon.

Diffraction

With diffraction, the light passes through a narrow aperture and spreads out.

Imagine a camera. The camera has an aperture, which determines how much light will enter into the lens. If more light tries to come in than the lens allows, the light will bend around the edges. When light bends around the edges, it can cause interference between the light waves, producing a halo effect.

But this doesn’t just happen with cameras. This can happen with our eyes too. Our pupils determine how much light comes in. If it’s dark, they dilate, and if it’s light, they shrink. After the light enters the pupil, it interacts with the lens: a round and flat part of the eye right behind the pupil.

Seeing the Lights

When we look at Christmas lights, our eyes take in a certain amount of light, and the rest bends around the edges of our pupils.

Unfortunately, the halo effect in Christmas lights is becoming a thing of the past. LED lights are starting to replace the old two-inch bulbs. These are more energy efficient, so they don’t put out as much light. Light probably won’t bend as much around the edges of your pupils, thus diminishing the chance of the halo.

As you look at lights this holiday, and as you remember the reason for the season, remember that your eyes let you see the world. So you’ll want to take good care of them. One simple way is by scheduling regular checkups with your optometrist.

This holiday, celebrate friends and family, and be thankful for your eyes!